The present embodiments relate to a magnetic resonance system with position-dependent slew rate limitation.
Exemplary modification of pulse sequences is provided, for example, by US 2010/0308829 A1.
In magnetic resonance systems, the gradient system switches gradient fields in order to encode the detected magnetic resonance signals in the spatial domain and/or in the frequency domain. In this process, the speed with which field changes are generated (e.g., slew rate) is directly linked to the image quality achieved and the measuring time required to perform the measurement. The faster the field changes are performed, the shorter the resulting measuring times. Distortion and artifacts may also be reduced. The prior art therefore attempts to perform field changes as quickly as possible.
However, the gradient fields that vary over time induce currents in the human body. These currents may cause peripheral nerves to be stimulated. Experts refer to this as peripheral nerve stimulation (PNS). In extreme instances, the gradient fields that vary over time may even cause stimulation of the heart muscle. Such stimulation may result in serious damage to health and in some circumstances even death for the patient. There are therefore statutory limit values that magnetic resonance systems may not exceed during operation. Compliance with such limit values is provided by a monitoring function that is implemented in the control facility of the magnetic resonance system. This monitoring function is often referred to as the gradient system watchdog (GSWD). The monitoring function implements an operating method.
The induction of currents and the corresponding stimulation of peripheral nerves are a function of different factors. One influencing factor is the design of the gradient coil system. A further influencing factor is the pulse sequence. The position of the patient or the examination object within the examination volume also plays a significant role. For example, the stimulation of peripheral nerves for a defined gradient pulse is greater when the ribcage of the patient is located in the isocenter of the magnetic resonance system than when the feet of the patient are located in the isocenter.
In the prior art (e.g., the abovementioned US 2010/0 308 829 A1), the worst possible scenario is always assumed. A worst case approach is therefore taken.